A Zimbra license is required in order to create accounts..
The Global Settings, License tab shows the current ZCS license status, including:
Company name for which the license was purchased.
License Type. Regular means that the ZCS license has been purchased for the number of accounts displayed in the Accounts Limit field. Trial means that a temporary licenses is being used. Trial licenses expire after 60 days.
Issue Date. The date that Zimbra created your license.
Effective Date. The date the license subscription begins.
Expiration Date. The date when your Zimbra license expires. When the ZCS license expires, you have 30 days to renew the license with Zimbra. Starting 30 days before the license expires, when you log on to the administration console, a reminder notice is displayed.
Accounts Limit. The maximum number of accounts you can create and the number of accounts created are shown.
The Update License button is on the toolbar of the License tab. The Update License opens a license wizard that installs the Zimbra license you receive from Zimbra.